DB RAC

In Real Application Clusters environments, all nodes concurrently execute transactions against the same database. Real Application Clusters coordinates each node's access to the shared data to provide consistency and integrity.
Benefits: Improve response time, Improve throughput, High availability and Transparency

A RAC System, by providing multiple instances (i.e., hosts and its associated resources) access the same database, creates multiple database computing centers and extends high availability and scalability.

A RAC Cluster database uses a SAN or other network storage device as a Shared System Disk. A RAC system must use a cluster file system or a raw partition, where any server can read or write to any disk in the shared disk subsystem. This allows access to all data files, control files, and redo and rollback (undo) areas by any instance. This ability to access all disks, allows instance recovery after an instance failure has occurred. All surviving nodes automatically absorb the failed instance’s tasks until the failed instance is brought back online, at which time it is fully synchronized and restored to service automatically.

A RAC cluster provides for automatic shared execution of Oracle applications. This means that for any Oracle instance application, all queries and other processing are automatically shared among all of the servers in the RAC cluster.

The sharing of application processing by all servers in the RAC cluster leads to automatic load balancing across all cluster members. The ability of a RAC cluster to provide shared application execution and automatic load balancing, leads to the true scalability of applications without code or data changes.